Anion Effect on CO₂ Electrochemical Reduction in Acidic Media

The CO₂ electroreduction reaction (CO₂RR) in acidic media offers a promising route to convert renewable electricity into value-added chemicals with high carbon utilization, yet its selectivity is severely limited by competing hydrogen evolution reaction. Despite extensive investigations into cation effects in acidic CO₂RR, the role of anions remains underexplored. Here, we systematically investigate the effect of three common electrolyte anions-Cl^-, SO₄ ^2-, and H₂PO₄ ^--on acidic CO₂RR. Theoretical studies suggest that Cl^- induces the most negative surface charge on Cu relative to SO₄ ^2- and H₂PO₄ ^-, which enhances C-C coupling and thereby improves selectivity toward C₂₊ products. Electrochemical evaluation shows that Cu catalysts in Cl^--containing electrolytes deliver the highest C₂₊ selectivity, surpassing those in SO₄ ^2- and H₂PO₄ ^- environments. Specifically, we achieve a peak C₂₊ Faradaic efficiency of 82% at 300 mA cm^-2. The promotional role of Cl^- is further observed on Ag catalysts in acidic CO₂RR. In situ Raman spectroscopy reveals that Cl^- enhances the adsorption of *CO intermediates on Cu surfaces, thereby promoting C-C coupling and C₂₊ product generation. This work reports an effective anion electrolyte engineering strategy to enhance CO₂RR performance under acidic conditions.